The measurement of fluorescence quenching or enhanced fluorescence emission when metal ions are bound to these fluorogenic ionophores is more accurate than measurements based on chromogenic phenomena. This is because fluorescence measurements are made against a dark background, whereas chromogenic methods require detection of absorption maxima or changes in absorption coefficients. Among the fluorogenic ionophores reported in the literature are those described by Nishida, et al. "Fluorescent Crown Ether Reagent For Alkali and Alkaline Earth Metal Ions," Chem. Lett., pp. 1853-1854, (1982), by Kenneth W. Street, Jr. and Shelly A. Kraus in "A New Metal Sensitive Fluorescence Reagent," Anal. Lett., 19 (7 and 8), 735-745 (1986), and by A. P. deSilva et al., "Fluorescent Signaling Crown Ethers: "`Switching On` of Fluorescence by Alkali Metal Ion Recognition and Binding in situ" J. Chem. Soc., Chem. Commun. 1986, 1709-10. However, all the above ionophores suffer from a disadvantage in that they are pH dependent, and can function only at a pH much higher than that of normal body fluid, and hence cannot be used for in vivo applications.